This type of linear motor has a magnetic field array comprised of first and second arrays of magnets. The magnets of each array are coupled magnetically by way of a ferromagnetic bar for providing a magnetic field of a selected geometrical pattern. The ferromagnetic bars are parallel to one another. Typically the linear motor also has a solid state commutated armature mounted for relative axial movement between the two bars and providing thrust when energized by current in a selected manner.
Many linear motors of the type described above are disclosed in the patent literature. Three examples of such linear motors are those shown in U.S. Pat. No. 4,151,447 entitled LINEAR MOTOR and issued Apr. 24, 1979 to Johann von der Heide and Georg F. Papst; U.S. Pat. No. 4,187,453 entitled ELECTRIC MOTOR DRIVE SYSTEM and issued Feb. 5, 1980 to J. Kirkwood H. Rough; and U.S. Pat. No. 4,396,966 entitled LOW MASS ACTUATOR SYSTEM FOR MAGNETIC RECORDING DISKS WITH HALF STRENGTH END POLES and issued Aug. 2, 1983 to Robert A. Scranton, David A. Thompson and Thomas K. Worthington. One problem common to all of the linear motors described in the above cited U.S. Patents is their inefficient use of the space available within their magnetic fields for generating thrust with the current in their armatures. In two cases, a portion of the available magnetic field is cut by current flowing the wrong way through a portion of the armature. Thus, the applied current partially generates reverse thrust. The other case shows an armature winding having a very low conductor stacking factor and hence even poorer efficiency.
U.S. Pat. Nos. 4,151,447 and 4,396,966 show an array of magnets mounted on each of two parallel ferromagnetic bars wherein the magnets are sequentially oppositely poled and the bars are mounted with respect to one another such that each magnet of each array of magnets is positioned opposite an oppositely poled magnet of the other array of magnets. U.S. Pat. Nos. 4,151,447 and 4,396,966 also show two coils spacially positioned in quadrature with respect to the arrays of magnets wherein each coil is wound such that when one side of the coil is positioned between a first pair of magnets the other side of the coil is positioned between an adjacent pair of magnets. Current is forced to flow through each coil at all times. The direction of current flow in either coil is reversed as that coil passes between the magnets of either array. Thus the current flowing in that coil is wasted at that position because it is distributed equally between opposing magnetic fields and has a net contribution of zero thrust. For other positions a portion of the current flowing in each coil is partly wasted depending on its spacial relationship with the arrays of magnets.
Also, any back e.m.f. present due to relative velocity between the armature and the magnetic field opposes only the net thrust producing portions of the current flowing in each coil. Since the control shown in U.S. Pat. No. 4,151,447 is a voltage control, the coil of the linear motor disclosed therein which produces the least thrust will always have the larger current. When such a coil is positioned between the magnet poles at the location where the direction of current flow is reversed, the net back e.m.f. is zero and the current is limited only by the series resistance of the coil and its drive amplifier.
U.S. Pat. No. 4,396,966 additionally shows end magnets of half strength. This is said to have the advantage of reducing leakage flux emanating from the linear motor. Also, it is evident that since only half of the flux from each magnet flows between magnets in the ferromagnetic bars then the ferromagnetic bars of the linear motor of U.S. Pat. No. 4,396,966 may be made half the size of the ferromagnetic bars of the linear motor of U.S. Pat. No. 4,151,447.
U.S. Pat. No. 4,187,453 shows two serpentine format windings formed via printed circuit techniques and placed in quadrature with respect to arrays of oppositely poled magnets similar to those of U.S. Pat. Nos. 4,151,447 and 4,396,966. The linear motor of U.S. Pat. No. 4,187,453 operates as a stepping motor. However, that linear motor could be controlled as a servo motor, as explained above, although it additionally shows a very low conductor stacking factor, as also explained hereinabove. It is cited herein both because it illustrates a method of constructing a multi-poled linear motor armature and because it illustrates a "moving field" configuration of linear motor wherein the linear motor has an elongated armature and a foreshortened magnetic field array.
What is needed is a linear motor having an ironless multiphase armature wherein no portion of the current flowing through any of the coils of the ironless multiphase armature generates reverse thrust. Further, it is desired that the coils of the ironless multiphase armature possess a high conductor stacking factor. It would also be desirable to provide a "moving field" configuration of a linear motor having an improved conductor stacking factor. Also, what is needed is a solid state switching arrangement for commutating the coils of the multiphase armature of the linear motor wherein all current conducting coils of the armature are connected in series so that each current conducting coil carries the same current.